Substrate binding to Candida tenuis xylose reductase during catalysis
Candida tenuis xylose reductase (CtXR) is studied by in situ NMR, saturation transfer difference (STD) NMR, and molecular docking with respect to its substrate and coenzyme binding in ternary complexes. The natural substrate Xyl as well as Glc and methyl-glucosides preferentially bind as α-anomers of the pyranose forms. These α-anomers are transformed faster, predominately leading to STD effects in the formed products, and can be better docked into the CtXR active site than the β-anomer. The reduction is initiated by α-Xylp ring opening prior to hydride transfer from NADH. Binding and transformation of unnatural 2,4-dichloroacetophenone is not as good, although it is reduced with very high catalytic efficiency. STD NMR indicates a reasonable amount to leave the ternary complex in unreduced form. The molecular docking calculation confirms this result, as only a couple of the investigated ternary complexes allow reduction of the substrates.